The color of a sample material can be accurately determined using a spectrophotometer. Spectrophotometers include a source of radiation, a device for isolating monochromatic light, and a photoelectric detector. Typically, the sample is illuminated with diffuse light from an integrating sphere which presently is disposed within the spectrophotometer. The integrating sphere receives radiation through an input port from a source such as a tungsten lamp; a reflective material, e.g., barium sulphate, reflects the radiation in multiple reflections to provide uniform diffuse illumination. An exit port is positioned to receive reflected radiation from the sample.
The reflected light is separated into its spectral components to provide reflectance measurements of the sample at each wavelength. Since the color of the sample itself causes a diminution and discoloration of the illumination within the sphere, a reference signal must be provided after the sample is positioned at the sample point. The sphere thus is also provided with a reference port positioned to receive light reflected from the wall of the sphere. Reflected light from the sample is compared to that from the wall to determine the reflectance over a number of wavelengths, e.g., 400-700 nm: the two signals are ratioed over the wavelengths to provide a signal proportional to the sample reflectance.
The device for isolating monochromatic radiation, such as a monochromator, contains a prism or defraction grating which disperses the light into its component wavelengths. The light is passed through an array of photodetectors each of which measures a different narrow frequency band of the radiation dispersed by the monochromator.
There are a number of applications, however, where it is not possible or convenient to position the sample at the sample port of the integrating sphere within the spectrophotometer. In the area of dentistry, people require replacement teeth for one or more of their natural teeth. Presently, a dentist attempts to match the characteristics of the natural tooth using shade guides which are tooth-shaped plastic or ceramic pieces attached to a short handle. The dentist visually compares the color of the tooth with different shade guides until an optimum match is obtained. The dentist then provides the guide number of the optimum shade guide to the technician who produces the replacement tooth using a formulation related to that guide number.
However, the guides do not cover the complete range of shades encompassed by natural teeth. The guides are not structured in a systematic way, e.g., the Munsell color order system which categorizes a color based on its hue, value and chroma. Further, natural teeth have a translucency and fluorescence which the replacement teeth rarely replicate. Moreover, the initial color measurement of the tooth is estimated visually rather than quantitatively measured by instrument. The type of illumination within the room, partial color blindness of some dentists, and other factors often diminish color quality of the restoration.
Other applications of color measurement include paint matching. Even if a customer knows the original formulation of the paint applied to a surface such as an interior wall, degradation over time fades the paint so that the original formulation no longer matches the present condition of the paint. Presently, a sample of the point must be physically removed from the wall to be analyzed by a spectrophotometer.